Hi, it's not only leg exercises. But also core needs to be strengthen. Plank, side plank push ups, bridge will be good start. You can do leg workouts twice a week in winter with 2 or 3 days break. Squats, lunges and plyometrics exercises will help with speed and prevention for cramps on bike. Interval high intensity on spin bike in winter months also helps to get stronger and fitter. Cycling is a specific sport so gym training helps but you really need the time on bike or indoor trainer.
